1. Field of the Invention
The present invention relates generally to article monitoring devices, and in particular to a system for detecting the removal of an article from within the vicinity of a base unit.
2. Description of the Related Art
Many systems and devices have been developed in an effort to detect the passage of persons and articles from beyond predetermined boundaries. For example, U.S. Pat. No. 4,593,273 (the '273 patent) describes an out-of-range monitor and alarm system that may be used to alert an operator or attendant that a supervised person has walked beyond a prescribed distance. The system of the '273 patent includes a base unit which preferably includes a radio transmitter having an approximately known maximum range to a sensitive receiver within a mobile unit carried by the supervised person. The receiver is disposed to produce an output signal whenever the detected radio frequency (RF) signal from the base unit falls below a predetermined level. In response, a transmitter within the mobile unit sends a signal to the base unit, which in turn sounds an alarm indicating that the supervised person has exceeded the prescribed range from the base unit.
Notwithstanding the existence of this and other RF proximity sensing systems, there remains a need for sensing/alarm devices and systems configured for particular applications. For example, existing RF proximity sensing and other alarm systems would in many ways be unsatisfactory if used to monitor small, inexpensive articles, such as hand-held writing instruments.
The expense of equipping a pen or other writing instrument with an RF transmitter and receiver may also substantially exceed the cost of the writing instrument or the like being monitored. RF systems also require utilization of an antenna, which would be impracticable in applications involving small writing instruments. In addition, the considerable power requirements of RF systems tend to render battery life unacceptably low for such applications. Finally, RF systems are subject to regulation by the Federal Communications Commission (FCC), thereby further complicating the design process. In addition, since RF energy penetrates most walls and doors, these monitoring systems are not disposed to monitor the removal of articles from an enclosed area such as a room.
U.S. Pat. No. 4,870,405 provides an alternative to RF-type systems for monitoring objects such as writing instruments. Specifically, removal of an object from a base receptacle initiates a timing cycle. At the conclusion of the timing cycle, an alarm sounds at the base receptacle unless the object has been replaced. However, the fixed duration of the timing cycle precludes accommodation of different intervals of use of the object or writing instrument. In this regard the setting the timing cycle to be of a relatively low duration (to ensure prompt notification of unauthorized object removal) may result in undesired sounding of the alarm during extended periods of authorized use.
The advent of various personal electronic devices, such as so-called "personal digital assistants", has also created a need for an inexpensive article monitoring system. In particular, it would be desirable to provide a cost-effective way to monitor the proximity of a stylus or other user input utensil to the electronic base unit. If the monitoring system were further configured with some form of alarm, the likelihood of inadvertent removal of the stylus from the vicinity of the electronic base unit would be substantially diminished. Given that certain personal electronic devices are being designed for wireless communication with a host system, the potential for radio interference renders RF-type monitoring systems even less preferable for such applications.
Accordingly, a need in the art exists for an article monitoring system which allows for flexible periods of authorized use, and which avoids the aforementioned shortcomings of RF sensing and detection techniques.